1. Field of the Invention
The present invention relates in general to a speed changing gear device in an automatic transmission for motor vehicles such as automobiles, and more particularly to such a speed changing gear device which is equipped with a main gear assembly and an auxiliary gear assembly which conduct respective speed changing actions, to provide a plurality of operating positions.
2. Discussion of the Prior Art
A known speed changing gear device used in an automatic transmission for a motor vehicle has a plurality of planetary gear units, each of which includes a sun gear, a ring gear, and a carrier rotatably supporting at least one planetary pinion meshing with the sun and ring gears. One of the three elements of each planetary gear unit, that is, the sun and ring gears and the carrier, serves as an input element which receives a rotary motion to be transmitted by the gear unit, and another element serves as an output element which is connected to another planetary gear unit or output member of the transmission, while the rest of the three elements is held stationary or inhibited from rotating. Thus, the planetary gear unit is capable of transmitting a received rotary motion from the input element to the output element, in one of opposite directions, so that the output speed is reduced or increased with respect to the input speed.
A plurality of the thus constructed planetary gear units may be assembled together to constitute the speed changing gear device, so that the individual planetary gear units perform respective speed changing actions, since each planetary gear unit is capable of conducting an independent speed changing action as described above. In this type of speed changing gear device, one of the planetary gear units (the first planetary gear unit) receives a rotary motion from the input member of the transmission, and the speed of the rotary motion is first changed during transmission thereof through the first planetary gear unit. Thereafter, the speed of the transmitted rotary motion is further changed during transmission thereof through another planetary gear unit next to the first planetary gear unit. Alternatively, a plurality of planetary gear units may be assembled together such that the appropriate elements of the planetary gear units are connected to each other so as to permit these planetary gear units to cooperate with each other to conduct a speed changing action. It is also possible that the speed changing gear device is constituted by a combination of a gear assembly consisting of a plurality of planetary gear units, and another planetary gear unit which is capable of conducting an independent speed changing action. The gear assembly indicated above is constructed such that the appropriate elements of the planetary gear units are connected to each other so as to permit these planetary gear units to cooperate with each other to conduct a speed changing action. Irrespective of the manner in which the planetary gear units are assembled, the speed changing gear device is required to meet various requirements or conditions. For example, the speed changing gear device should be small-sized in view of limited installation space in the transmission system for accommodating the gear device. It is also desirable that the gear device is capable of providing as many operating positions as possible for reduced shifting shocks, and that the number of frictional coupling elements such as clutches and brakes which must be operated when the transmission is shifted up or down is reduced to a minimum so as to facilitate control of the gear device for shiftings of the transmission and to minimize the amount of shifting shock.
Various speed changing gear devices have been developed to satisfy the requirements as described above. For example, laid-open Publication Nos. 51-48062, 54-132058 and 60-88252 of unexamined Japanese Patent Applications disclose speed changing gear devices having three planetary gear units for providing five forward drive operating positions.
The speed changing gear device as disclosed in one of the above-identified publications No. 51-48062 has two single-pinion type planetary gear units and one double-pinion type planetary gear unit. This gear device is constructed such that the appropriate elements of these three planetary gear units are permanently connected or fixed to each other so as to permit the planetary gear units to cooperate with each other to perform a desired speed changing action. While five forward drive positions can be established by this speed changing gear device, the smallest speed reduction ratio available on this gear device is "1", that is, the gear device is not able to provide an overdrive position whose speed reduction ratio is lower than "1". Further, when the transmission is shifted up or down, the speed changing gear device requires disconnection of one element of the gear device from the input member of the transmission, and connection of another element to the input member, and at the same time requires releasing the currently fixed element from a stationary member and fixing another element to the stationary member, whereby a considerable amount of shifting shock is induced. Therefore, the disclosed speed changing gear device requires intricate regulation of the timings at which the frictional coupling elements are engaged or disengaged, for reduced shifting shocks. In this speed changing gear device wherein the appropriate elements of different planetary gear units are fixed to each other, it is practically impossible to convert this gear device having five forward-drive positions into those having four and three forward-drive positions. Thus, this gear device suffers from relatively low versatility and cannot be so widely used for various types of transmission.
On the other hand, the speed changing gear device as disclosed in the above-identified publication No. 54-132058 has a combination of a Ravigneaux type planetary gear assembly and a single-pinion type planetary gear unit. Some operating positions of the gear device are established while the gear assembly and the gear unit are operated independently of each other. Specifically, the speed of the input rotary motion is first changed during transmission thereof through the Ravigneaux type planetary gear assembly, and the speed of the rotary motion transmitted from the gear assembly is further changed during transmission thereof through the single-pinion type planetary gear unit. In the other operating positions, the appropriate elements of the gear assembly and gear unit are connected to each other, so that the gear assembly cooperates with the gear unit to perform a desired speed changing action. In this arrangement, it is practically impossible to eliminate the single-pinion type planetary gear unit and thereby convert the present gear device having five forward-drive positions into those having fewer forward-drive positions. Thus, the speed changing gear device disclosed in this publication also suffers from relatively low versatility. Further, the present device requires a total of three frictional coupling elements to be operated for engagement or disengagement, when the transmission is shifted up from the 3rd-speed position to the 4th-speed position. Consequently, the shifting to the frequently used position (4th-speed position) requires intricate control of the three coupling elements of the gear device, so as to avoid increased shifting shocks.
The speed changing gear device as disclosed in the above-identified publication No. 60-88252 is constituted by a first, a second and a third single-pinion type planetary gear unit, for providing five forward-drive positions including an overdrive position and one reverse position. In this gear device, the 2nd-speed position is established such that the three planetary gear units conduct respective speed changing actions one after another during power transmission through the gear device. When the gear device is placed in the other positions, however, the mutually connected three planetary gear units cooperate with each other to conduct the relevant speed changing action. In this case, too, it is practically impossible to convert the present gear device into those having fewer operating positions. Further, the speed changing gear device disclosed in this publication suffers from a relatively large gear ratio .rho. of the planetary gear units, which unfavorably increases the radial size of the gear device.
The conventional speed changing gear devices as described above are constructed such that the appropriate elements of the planetary gear units are connected to each other, so that the gear units as a whole contribute to a speed changing action for a desired operating position. Therefore, these planetary gear units cannot be radially spaced from each other and must be disposed in series coaxially with each other. Such a speed changing gear device can be suitably installed on the front-engine rear-drive type of vehicle, but not on the front-engine front-drive type of vehicle wherein the power transmission system must be accommodated in a casing having a relatively limited axial length.